In impact devices of the type driven by a rotary motor it is necessary to convert the rotary motion of an output shaft to reciprocating motion of a body, usually including a resilient coupler, to drive a ram into impacting against an output tool. In such prior devices the generation of high impacting forces in relatively light weight structures is accompanied by inefficiency of energy conversion caused by unnecessary heating and unwanted harmonic vibrations which seriously waste energy and reduce service life. In particular, considerable such extraneous harmonics are generated by the structural means incorporated to change rotary shaft motion to reciprocating body motion.
In most such prior art devices the rotary-to-reciprocative motion converter includes a connecting rod operatively connected to a crankshaft to reciprocate a body in one of two ways. In the most common of these, the connecting rod is connected with its principal axis in a direct line between a crankpin on a crank on the crankshaft and a wrist pin on the reciprocating body, the principal axis of the connecting rod during each cycle of operation thereby assuming angles up to a maximum of 10 to 15 degress each side of the geometric plane passing through the crankshaft rotational axis and the wrist pin axis. In applicant's U.S. Pat. No. 4,014,392 (3-29-77) the extraneous harmonic vibrations generated by such connecting rods are significantly reduced by incorporating structures for which the aforementioned maximum angles are reduced to less than four degrees.
In other such impact devices a rotary-to-reciprocative motion converter as shown in FIG. 1 also includes a connecting rod 12 operatively interconnecting a crankpin 14 rotating on crankshaft 18 to reciprocate body 15. The extraneous harmonic vibrations generated by angular positioning of the connecting rod interconnected as described hereinabove is avoided by the arrangement illustrated in FIG. 1 by fitting connecting rod 12 to slide axially in bushing 24 and by incorporating slotted connecting rod end 10 with slot sides 20 and 22 in which crankpin 14 is pivoted as crankshaft 18 rotates. (Connecting rod 12 with slotted end 10 is sometimes referred to as a Scotch yoke.) Thus, connecting rod 12 moves in reciprocation ideally without angular positioning during operation. In practice, however, such devices are seriously deficient because the spacing between slot side 20 and slot side 22 must be greater than the outside diameter of crankpin 14 for crankpin 14 to slide in the slot with a practical degree of freedom from friction. Accordingly, during operation there is a tendency for crankpin 14 to impact from side to side as crankshaft 18 rotates because the reactive load on connecting rod 12 reverses during each cycle of operation. Such impacting generates extraneous harmonics of considerable amplitude and induces additional wear on the related surface and crankpin which increases the impacting effects described. When compounded with extraneous harmonics in the reactive forces from body 15, and lateral motion of the connecting rod 12 within the geometrical clearance between connecting rod 12 and bushing 24, necessary for a sliding fit therebetween, significant extraneous harmonic vibrations are generated with resultant unnecessary waste of energy and shortened service life. Connecting rods operatively interconnected in the manner just described are disclosed in U.S. Pat. No. 1,052,823 (2-11-13).
In the third type of rotary-to-reciprocative motion converter used in prior art devices a shaft rotates a cam surface which slides against a second surface. Such devices accordingly are deficient and because of irregular or non-linear motion have severe wear, relatively short service life and generate serious extraneous harmonics.